D.Yarosh, R.S. Day, III, and others have shown that approximately 20% of human tumor lines and viral transformed lines are hypersensitive to alkylating agents due to an apparent absence of 06-methylguanine DNA methyltransferase (06MT); this phenotype has been designated mer-. This enzyme removes alkylation damage at the 0-6 position of guanine but not at other sites in DNA. We and others have isolated 06MT cDNA clones. Our clone has been used as a probe to measure mRNA levels and has also been used to synthesize recombinant protein for antibody development. We have found previously that 06MT mRNA was markedly reduced in all the mer- tumor cells lines examined. The level of 06MT mRNA varied by 10-fold in mer+ cell lines and correlated with known levels of the protein in particular cells. In collaboration with D. Yarosh, 06MT mRNA levels and 06MT activity have now been determined in biopsy specimens of patients with lung and brain tumors. The activity varied widely in tumors from different patients, but was similar in tumor and adjacent normal tissue from the same patient. In general, relative 06MT mRNA levels correlated with activity, but in several lung biopsies the activity was quite low while mRNA levels were not. This indicates that 06MT expression may also be controlled at the translational and/or post-translational levels. Our findings may have important implications in cancer therapy where agents such as BCNU are used.